With good mechanical characteristics, electric properties, dimensional stability, heat resistance, transparency, and chemical resistance, polyester films have been used in a variety of fields including various industry materials, packaging materials, and magnetic materials. Hard coat films with improved scratch resistance and wear resistant produced by forming a hard coat layer over the surface of a polyester film to increase its surface hardness and abrasion resistance in recent years have been in greater demand in the market to meet the increasing production of their intended applications including surface protective films and reflection films in flat panel displays such as liquid crystal display and plasma display as well as touch panel, display board, name plate, window covering, and surface decorating material, and demands for them are expected to continue growing in the future.
Such polyester films with a hard coat layer can be produced by laminating a polyester base film with a hard coat layer. However, polyester base films directly coated with a hard coat layer have some problems including poor adhesion and interference fringes resulting from a difference in refractive index between the hard coat layer and the polyester base film to degrade their appearance, and studies have been made to solve this problem by forming a thin film with specific constitution to cover the polyester base film. (Japanese Unexamined Patent Publication (Kokai) No. 2007-253512 and WO 2009/145075)
For hard coat films used in the latest mobile devices, in particular, hard coat layers of various designs with a high degree of multifunctionality have been developed to suit different applications. For example, some hard coat layers contain an ultraviolet absorber to prevent color degradation from being caused by ultraviolet rays (Japanese Unexamined Patent Publication (Kokai) Nos. 2005-335163 and 2010-083029), some having an antifouling function to prevent dirt from being attached (Japanese Unexamined Patent Publication (Kokai) No. 2010-152331), and some having a moldability to fit to the shapes of members (Japanese Unexamined Patent Publication (Koaki) No. 2009-220280). As described above, as hard coat layers are diversified, there is an increased need for a variety of lamination layers to achieve strong versatile adhesion between polyester base films and a wide range of hard coat layers of different designs, but conventional techniques have only a limited adhesion capability and require a modified lamination layer to suite the design of a particular hard coat layer. This can result in decreased productivity and increased cost, or in some cases, it is necessary to make modifications to a hard coat layer even if they may cause deterioration in its functionality. In cases where ultraviolet absorptivity is required, a large amount of an ultraviolet absorbent has to be added to a thin hard coat layer, but it can cause a noticeable decrease in adhesiveness, leading to the problem of failure in maintaining adequate performance. In addition to improving the normal initial adhesive strength, furthermore, a high degree of durability is also required to maintain a high adhesive strength in an high-temperature, high-humidity environment, and there is a call for a polyester film that has both these properties as well as high adhesive strength.
It could therefore be helpful to provide laminated polyester film that can maintain versatile and durable adhesion to various hard coat layers with different functions including ultraviolet absorption, and further serves to develop good appearance with suppressed interference fringes.